1. Field of the Invention
The present invention relates to an automatic alignment system for use with a digital modem which operates to automatically control one or more alignment points in the modem to optimize modem operation on the basis of a performance indicator measurement.
2. Description of the Prior Art
A typical data transmission system, such as those used in satellite communications, includes a digital modem which normally needs to be adjusted on site for obtaining optimum performance characteristics, particularly at start up. These performance characteristics are measured as an error rate in the digital modem, which measurement may be accomplished in various ways. One way is to compare the expected value of the incoming signal with the actual received signal. The result of this comparison is then reflected as a bit error rate (BER). However, this requires a prior knowledge of the value of the incoming signal.
Another method of determining the BER involves a "link performance indicator circuit" (LPI), of the type which is the subject of copending application Ser. No. 138,456 filed Apr. 8, 1980, and which is assigned to the assignee of the present invention. This link performance indicator circuit provides an estimate of the bit error rate by developing "pseudo-errors" as a result of a comparison of the incoming signal to an average value of the incoming signal. These "pseudo-errors" provide a relationship to the real errors based on known relationships, and therefore, does not require a prior knowledge of the value of the incoming digital signal.
Previously, the alignment of the digital modem was accomplished by the use of alignment points which were manually adjusted for optimum performance. This alignment of each point was accomplished on an individually adjusted basis by trained personnel in order to attain a minimum reading of the measure of the performance, such as provided by the LPI signal at the output of the above-mentioned link performance indicator circuit. However, all of the alignment points typically interact with one another, and therefore, several passes through all the alignment points are required before the adjusting operation provides assurance of good alignment. Such a system requires highly-trained personnel and takes an inordinate amount of time to properly fine tune the system.
Prior art attempts at effecting automatic changes in the performance of a data transmission system have been addressed to error correction systems which treat burst errors and random errors in a somewhat effective manner through forward error correction, as illustrated in the patent to Lutzker, U.S. Pat. No. 3,657,700, and the patent to Abramson, et al., U.S. Pat. No. 3,506,961. However, none of the prior art systems are based upon the automatic adjustment of alignment points of the modem or serve to properly align the unit prior to effective use.
The patent to Lutzker, U.S. Pat. No. 3,657,700, discloses the use of an error correcting data communication system which has the capability of varying the error correction potential without varying the codes or increasing the redundancy of transmitted data. This is accomplished by a buffer, having several stages, which is utilized during the transmission of processing control signals in order to request a change in the value of an adaptive parameter. When a correctable error occurs, a bit is read out of a register into an adder in order to correct the bit which is then read out of a buffer. The output of the adder is read into a control signal reader which generates an output to a control unit indicative of the received control signal. In turn, the control unit generates an appropriate control signal or signals which effect the necessary correction of the error.
Another example of error correction in a digital transmission channel is shown by the patent to Merlow, U.S. Pat. No. 3,733,585, which effects alignment of the signal to be used to check the system. When a loss of alignment has occurred, the signal on the output lead is inhibited and replaced by a second output signal which is passed to a pattern acquisition control element. This in turn provides signals to the output terminal in order to delay the counting action of a clock counter. This delay caused by the clock counter causes a shifting of control pulses relative to the incoming digit stream. Once again, this system is directed toward a rechecking of data for errors which occur in the system and must be corrected prior to retransmission.
It is therefore an object of the present invention to provide an automatic alignment system in which the control voltage applied to one or more alignment points in an electronic circuit may be automatically adjusted on the basis of a performance indicator signal representing the performance quality of the circuit.
It is another object of the present invention to provide an automatic alignment system of the type described which avoids or otherwise overcomes the inherent deficiencies of known systems of this type and accomplishes this in a simple and economical manner.